Guarding is an important aspect of virtually any low-current measurement. Electrical instrument manufacturers typically employ any of a number of conventional techniques to isolate the impedance of a desired measurement from all currents and impedances that are not part of the required measurement. Guarding is essentially a special case of shielding to prevent noise or unwanted currents from becoming part of the measurement by preventing coupling to the measurement leads. Guarding additionally prevents direct current (DC) and, ideally, alternating current (AC) normal mode currents from electrically coupling to the measurement terminal.
In conventional electrical test and measurement systems, the guard is typically driven to exactly the same voltage as the measurement terminal by the test and measurement device. This guard should be present at all locations around the measurement terminal except immediately at the intended measurements. However, several problems arise in the process of driving the guard to the same voltage as the measurement terminal. For example, a feedback path is present from a guard amplifier to the measurement terminal, which is the input of the guard amplifier. Because this amplifier has a feedback path, maintaining frequency stability is required but conventional attempts to do so undesirably change the circuit phase characteristics.
Conventional systems generally involve the addition of resistance in series with the guard amplifier of the guard drive circuit to provide frequency stability, but this decimates the guard's usefulness as a shield. Further, such systems allow the guard voltage to deviate from the measurement node voltage under transient conditions, e.g., due to the series guard resistance, and are thus not sufficient for typical fast low-current measurement systems. Low current measurements require some sort of technique to manage or control which currents or impedances are measured, and which currents are excluded.
Even though the guard amplifier of the guard drive circuit may return to stable operation, provided the resistor is large enough, it is at the expense of guard bandwidth, and a higher impedance guard output and the guard will lag behind the measurement terminal, allowing guarded dielectrics to charge and discharge during transients. Also, longer cables, which generally have more dielectric to charge up, will see undesirably slow guard response and settling time, which is unacceptable performance.
Thus, there remains a need for improved techniques for guard drive circuit integrating with electrical test and measurement equipment.